Although the G1 has accelerometers changing the display from portrait to landscape, you need to slide the keyboard our or got to compass mode in street view to enable it. Thanks to the open source nature of the Android platform, it is only a matter of time until a dev comes up with a hack that will rotate automatically when turned on the side.
Below is a video posted by nuuneoi demonstrating a working G1 utilizing the accelerometer to auto-rotate when the handset is turned. It is argued that this feature is a part of the original SDK and developers need only to unlock ability but if this is the case why didn’t they make this a built in feature?
[nuunneoi]